|
Continued from Intestine and Villi
The capacity of the Intestinal Canal depends upon
the nature of the bird's food. In order to compare its length in
different forms we require a unit by which to arrive at its relative proportions.
The length of the whole vertebral column, or even the distance from the
tip of the bill to that of the tail, has been frequently used; but this
gives only faulty results, since the length of the neck is obviously not
correlated with that of the intestine. Numerous measurements and
comparisons led 19th Century naturalist Alfred Newton to adopt as the unit
the distance from the first thoracic vertebra (i.e. from the root
of the neck) to the anus, and thus the quotient of the absolute length
of the intestine from the pylorus to the anus is the relative length of
the gut. This relative length is very constant in a species, and was considered
to often give results of considerable taxonomic value.
Of course "short-gutted" and "long-gutted" are
arbitrary expressions; but, if we assume that a relative length not exceeding
5 indicates a short, and one of more than 8 a long gut, we find that the
Intestinal Canal is very short in all purely frugivorous and insectivorous
birds, while it is very long in those which live upon fishes, carrion,
grain, and grass. It must, however, be remarked that, according to the
nature of the food, a short intestinal canal is often compensated by its
width either wholly or in part, as of the rectum, or by the presence of
large caeca. Consequently
all these points had to be considered in using the features of the intestine
for taxonomic purposes. Caeteris paribus,
the relative length of
the canal is as good a character as many others, and occasionally by it
alone closely-allied species can be determined. The table below shows the
measurements of the intestine in a few forms of Newton's day. For
more information Bronn's Klassen und Ordnungen des Thier-Reichs Vogel,
pages 590-661 and 700, gives the respective measurements of nearly
400 birds.
|
Absolute
Length |
Length
of |
|
of |
Intestinal
Canal. |
|
1
caecum./ Rectum
|
Absolute |
Relative |
|
|
|
|
cm. |
cm. |
cm. |
|
| Struthio
camelus |
70 |
820 |
1430 |
20 |
| Casuarius
indicus |
13 |
28 |
180 |
3-4 |
| Spheniscus
minor |
2 |
7 |
223 |
16 |
| Anser
cinereus, var. dom. |
24 |
18 |
260 |
12 |
| Procellaria
leachi |
0.2 |
1.5 |
29 |
5 |
| Ardea
cinerea |
0.5 |
10 |
212 |
10 |
| Gallus
bankiva, var. dom. |
17-20 |
8-11 |
136-170 |
8-10 |
| Syrrhaptes
paradoxus |
12 |
10 |
80 |
9 |
| Columba
Livia, var. dom. |
0.8 |
4 |
108-132 |
11-13 |
| Pandion
haliaetus |
0.3 |
9 |
300 |
18 |
| Astur
palumbarius. |
0.7 |
7 |
108 |
6 |
| Corythaix
persa |
0 |
- |
42 |
3-4 |
| Cypselus
apus |
0 |
- |
17 |
3 |
| Corvus
corax |
1.4 |
5 |
120 |
8 |
| Manucodia
atra |
0.5 |
3 |
29 |
2-3 |
| Passer
domesticus |
0.2 |
2 |
21 |
5-6 |
|
